Effect of diethylstilboestrol on the relationship between LH, PMSG and progesterone during pregnancy in the mare.

Two studies were conducted to determine the relationship between LH and progesterone and between PMSG and progesterone during pregnancy in mares. In the first, samples of jugular blood were collected daily from 7 mares from the first day of oestrus until Day 28 of pregnancy, and in the second, samples were collected weekly from 14 mares from Day 35 of gestation until parturition. In an attempt to prolong secretion of progesterone from accessory corpora lutea, 7 of these 14 mares were injected with increasing doses (2--10 mg) of diethylstilboestrol (DES) between Days 84 and 142 of gestation. The remaining 7 mares received injections of vehicle. Concentrations of LH, PMSG and progesterone in serum were determined by radioimmunoassay. From the onset of oestrus until Day 4 of gestation, serum concentrations of LH and progesterone were negatively correlated (r = 0.67, P less than 0.01), but from Days 5 to 28 a positive correlation (r = 0.80, P less than 0.01) was noted. Likewise, serum concentrations of PMSG and progesterone were highly correlated between Days 35 and 196 in mares injected with DES (r = 0.72, P less than 0.01) and the vehicle (r = 0.75, P less than 0.01). Injections of DES did not influence serum concentrations of LH, PMSG or progesterone, or affect the length of gestation. It was concluded that DES does not influence the maintenance of pregnancy in the mare.